Lead blast-furnace process



March 29, 1966 J. LUMSDEN 3,243,283

LEAD BLAST-FURNACE PROCESS Filed March 24, 1964 3 Sheets-Sheet 2 0.0 1 n I I 04 O-6 0-8 l-O I-Z L4 L6 I-8 SLAG/Pb FIG .3.

March 29, 1966 J. LUMSDEN 3,243,283

LEAD BLAST-FURNACE PROCESS Filed March 24, 1964 3 Sheets-Sheet 5 SLAG/Pb 25 I 1 l l a 0-4 0.6 0-8 l-O l-2 [-4 L6 L8 20 x L--- Y 0 I6 018 II M I l l- N o. .0 l2 |.4 I6 I- SLAG/Pb 8 Patented Mar. 29, 1966 3,243,283 LEAD BLAST-FURNACE PROCESS John Lumsden, London, England, assignor to The National Smelting Company Limited, London, England Filed Mar. 24, 1964, Ser. No. 354,338 Claims priority, application Great Britain, Jan. 6, 1961, 708/61; Oct. 9, 1961, 36,151/61 11 Claims. (CI. 7577) This invention relates to improvements in the process of smelting lead in a blast furnace to produce metallic lead and is a continuation-in-part of my application Serial No. 164,075, filed January 3, 1962, now abandoned.

In this known process most of the lead in the materials :charged to the furnace is present as oxidic lead compounds, such as lead oxide, basic lead sulphates, or lead silicate in slags arising from other metallurgical operations. Coke is generally used as fuel, and air is blown in near .the furnace bottom. Neither the air blast nor the charge is preheated before it is introduced into the furnace. Lead metal is tapped from the furnace bottom, together With a molten slag; under some circumstances, a copper-containing matte may be tapped also. The lead metal contains readily reducible metals such as silver, but the less readily reducible oxides, such as zinc oxide and iron oxide, remain unreduced and are contained in the slag.

The original raw material is not usually in a suitable physical and chemical state for charging to the blast furnace. The main primary source of lead is a lead sulphide concentrate, which must be oxidized before it is charged to the blast furnace. In modern practice, it is usual to sinter roast a lead sulphide concentrate, adding limestone and other materials to the sinter mix, so that, apart from the coke, most or all of the charge consists of sinter.

In the prior art, it has been found that materials of high lead content cannot be satisfactorily smelted in a blast furnace. If materials containing more than about 50% lead are charged to a blast furnace, it has been found that the furnace does not operate smoothly, the operating troubles being associated with the development of a high temperature at the furnace top and a tendency for the charge to form bridges across the furnace, with consequent uneven descent and hanging-up of the de scending charge. Consequently, it has been found neces* sary to incorporate in the furnace charge, not only the oxidized raw material, together with such fluxes as are necessary to form a slag of suitable composition with the non-reducible oxides necessarily present (as gangue in the lead concentrate and as ash in the coke) but also some of the slag run ft from the furnace, as a diluent to lower the temperature at the furnace top.

It is an object of the present invention to improve the fuel economy and increase the output of a lead blast furnace by making it possible to produce more lead, for the same amount of carbon consumed, than was possible according to the prior art.

It is a further object of the invention to make it possible to smelt charges of higher lead content than was possible according to the prior art.

It is a further object of the invention to improve the efliciency of operation of a lead blast furnace by con trolling the temperature and composition of the blast so that the fuel consumed is efficiently utilized for both gen erating heat for melting slag and providing reducing gases for reducing lead oxide.

I have discovered that, with the types of charges that have hitherto been smelted in a lead blast furnace (charges containing not more than about 50% lead), the use of preheated air blast enables the amount of lead to be smelted (for a given carbon consumption in the furnace) to be considerably increased, this increase beapproximately proportional to the degree of preheat up to a certain limit. So long as the degree of preheat does not exceed this critical limit (which is higher the higher the ratio of slag to oxidized lead in the charge) the use of preheated air results in a saving of total carbon consumed in relation to lead produced, even when allowance is made for the carbon required for preheating the air.

I have also discovered that when steam (water vapour) is added to a preheated air blast, high grade lead charges, containing considerably more than 50% lead, can be satisfactorily smelted. The concentration of water vapour required in the blast, to prevent operating troubles arising from a high temperature being generated near the furnace top, increases with reduction in the ratio of slag produced to lead as oxide in the charge. As the amount of water vapour thus required increases, the degree of blast preheat required increases also, and by increasing this degree of preheat up to suitable limits a considerable economy is attained in the carbon used per unit weight of lead oxide reduced to lead as compared with the prior art in which, with cold air as blast, extra slag is included in the charge.

Finally, I have discovered that a further economy in the carbon used per unit of lead oxide reduced to lead can be achieved by adding to the blast more steam than is required to prevent the temperature near the furnace top becoming so high as to cause hanging up of the charge (or by adding steam to the blast for charges that need no steam to prevent hot top conditions) and suitably further increasing the degree of blast preheat above the aforementioned limits, but the ratio of the carbon economy thus effected to the additional heat supplied in the blast is considerably less than this ratio for the preheat increases up to the aforementioned limits.

The oxygen-containing blast may consist of atmospheric air or any other mixture of oxygen and nitrogen, with steam (water vapour) either naturally present or deliberately added.

The invention consists in a method of operating a lead blast furnace in which materials containing lead oxide and carbon are charged at the top of the furnace, molten lead, containing readily reducible metals such as silver, is tapped from the bottom of the furnace and a molten slag of low lead content but containing the less readily reducible oxides present in the charge, is also removed from the bottom of the furnace, the weight of slag tapped being less than the weight of oxidized leadcharged, and a preheated blast containing both oxygen and water vapour is supplied to the bottom of the furnace, the degree of preheat of the blast being suflicient to ensure a low lead content of slag tapped from the furnace bottom, and the amount of water vapour being at least sufiicient to prevent an excessively high temperature being generated near the furnace top.

The blast may consist of atmospheric air with steam, or oxygen may be added to bring the oxygenmitrogen ratio up to a somewhat higher value than that of atmospheric air.

My discoveries of the efficient utilization of preheated blast, and of the favourable effect in some circumstances of adding steam to the preheated air blast are based on a new concept of the manner of operation of a lead blast furnace.

I postulate that in the lead blast furnace there are three zones; a slagging zone at the bottom of the furnace where slag is melted at a temperature around 1100-1200 C. at the expense of heat provided by combustion of carbon to a mixture of carbon monoxide and carbon dioxide, a middle zone where some of the carbon dioxide reacts with carbon to give carbon monoxide, and a reduction zone near the top of the furnace where lead oxide is reduced to lead by reactions that evolve heat.

From this model I will show in the following description under the heading (A) that in the hitherto used method of operating a blast furnace the carbon does not perform efiiciently its dual role of producing heat and acting as a reducing agent and under the heading (B) how this efiiciency can be improved by the operation of a blast furnace according to the invention.

I will also show how the loss of efliciency due to the necessity of recirculating slag can be reduced or eliminated.

(A) Operation of the conventional lead blast furnace In a lead blast furnace, the gases ascend in countercurrent with the descending charge. The sequence of processes taking place can be followed with respect to either the descending charge or the ascending gases. It is convenient first to consider the descending charge and then to study in more detail the various processes occurring as the gases proceed up the furnace.

As the charge enters the furnace it first becomes heated by the gases and any water contained in it is evaporated. When the temperature reaches 400500 C. lead oxide begins to be reduced by carbon monoxide:

While this lead oxide is being reduced, the temperature of the charge continues to rise, partly because it is still being heated by convection from the gases and partly because Reaction 1 generates heat. There is usually some sulphur present in the charge, partly as lead sulphide, and when the charge temperature rises to the region of 700 C., some sulphur dioxide is generated by reaction of lead sulphide with lead oxide:

Sulphur dioxide can be generated also by other reactions, for example, by reaction of lead sulphide with basic lead sulphate:

Only part of the sulphur in the charge is, however, removed as sulphur dioxide. Reactions 2 and 3 are proceeding in competition with reactions by which lead is being reduced, from lead oxide by Equation 1 and from basic lead sulphate by the reaction:

In addition any sulphur combined as calcium sulphate is not converted to sulphur dioxide to any important extent, but is reduced to sulphide by carbon monoxide. Lead is thus reduced to metal by several reactions, but chiefly by Reaction 1. At least at temperatures above 600 C., Reaction 1 proceeds rapidly, so that nearly all the lead is reduced to metal in a zone near the top of the furnace. For future discussion, this upper zone of the furnace, down to the level where nearly all the lead oxide has been reduced, will be referred to as the leadreduction zone.

If, by the time that all the lead oxide has been reduced, temperature equality has not been attained between gas and charge, the relatively rapid heat convection soon establishes thermal equilibrium at a temperature in the region of 1000 C. Thereafter, the temperature of the charge rises relatively slowly as it descends, heat exchange between gas and charge preventing any substantial temperature difference between them, until both their temperatures have reached a point at which molten slag is formed. What will be referred to as the middle zone of the furnace is bounded at its upper limit by the leadreduction zone and at its lower limit by the level at which formation of molten slag begins.

The third and lowest zone of the furnace will be referred to as the slagging zone. In this zone the coke burns to give a mixture of carbon monoxide and carbon dioxide. Near the tuyeres carbon dioxide is formed:

This carbon dioxide then reacts with carbon to give carbon monoxide:

CO C=2CO 6) Reaction 6 is endothermic, so that, as it proceeds, it brings the temperature down below the high values that may be attained locally near the tuyere tips. Further, Reaction 6 is rate-controlled, its rate falling sharply with falling temperature. Consequently, any intrinsically fast reactions proceed relatively rapidly compared with Reaction 6 and thus tend to approach a state of chemical equilibrium. Since all the lead oxide has been reduced before the slagging zone is reached, no such chemical reactions would take place if the charge contained only lead oxide and inert slag-forming materials. Most lead concentrates, however, contain some zinc and, as already mentioned, of the sulphur present in the charge, some reaches the lower parts of the furnace. As will subsequently be explained, the charge reaching the slagging zone contains, in addition to its original zinc and sulphur, additional zinc oxide and zinc sulphide formed by reaction of the vapours of zinc and lead sulphide with each other and with the furnace gases higher up in the furnace. If any large amount of zinc is present in the charge, both zinc oxide and zinc sulphide are present in the materials descending into the slagging zone. The reduction of zinc oxide by carbon monoxide is a rapid reaction so' that, in the slagging zone, approximate equilibrium is attained in the endothermic reaction:

In addition, some lead is vaporized according to its vapour pressure, and this lead vapour can react with zinc sulphide to give the vapour of lead sulphide by the endothermic reaction:

Pb (gas)+ZnS:PbS (gas)+Zn (gas) (8) Although lead sulphide is vaporized in considerable amounts (but, under normal circumstances, in less volume concentration than zinc), thermodynamic calculations show that liquid lead sulphide, as such, is not stable under the conditions prevailing in the slagging zone. Equation 8 is written as representing a direct mechanism by which the vapour of lead sulphide, together with zinc vapour, can be generated. Since equilibrium is attained in both Reactions 7 and 8, the extent to which lead sulphide is generated can be represented by Equation 7 subtracted from Equation 8:

With the lead reactant liquid, Reaction 9 is highly endothermic.

The condition that equilibrium is attained in Reactions 7 and 9 permits, at a constant temperature of slag melting, an arbitrary adjustment of the CO/CO ratio in the gas, with consequent variations of the concentrations of zinc (proportional to CO/CO and lead sulphide (proportional to CO /CO). The final gas may be regarded as generated by Reaction 5, which evolves heat, followed by Reactions 6, 7 and 9 all of which absorb heat.

The heat balance of the slagging zone of the furnace can be considered with the melting point of the slag as the reference temperature, which is the temperature at which the charge enters the slagging zone and at which the gases leave. Air enters the tuyeres at atmospheric temperature and the molten slag leaves the furnace at a temperature above its melting point. The only source of heat is then that generated in Reaction 5 at the slag melting point, from-which must be subtracted the heat required to heat the air up to this temperature. Heat requi-rements are for heat losses from the slagging zone of the furnace, and for melting the slag and heating it from its initial melting point to the temperature at which it leaves the furnace. The heat balance is then satisfied by the appropriate amount of endothermic Reaction 6 occurring, together with vaporization of lead and the amounts of the endothermic Reactions 7 and 8 required to attain equilibrium in these two reactions at the temperature at the top of the sl-agging zone. The heat balance is automatically satisfied in this particular way because Reaction 6 is slow compared with Reactions 7 and 9.

Other things (particularly heat loss per unit of carbon consumed, and zinc content and melting point of slag) being equal, the greater the ratio of slag to carbon, the less is the fraction of the carbon reacting to give carbon monoxide according to Reaction 6, since additional carbon monoxide means, not only additional heat absorbed in Reaction 6 but also in view of the increased CO/CO ratio, more of the endothermic Reaction 7, which is only partly compensated by less lead sulphide generated by the endothermic Reaction 9; at the CO/CO ratios of interest, the volume concentration of lead sulphide in the gas is usually less than that of zinc vapour (when the charge is of high zinc content). There are two reasons why the CO/CO ratio in the gas leaving the slagging zone cannot be allowed to fall below a certain level. Firstly, this might lead to too little carbon monoxide being available higher up in the furnace to reduce all the lead oxide. Secondly, the lead oxide content of the slag is determined by an equilibrium between slag, bullion and gases, according to the reaction:

Pb+CO =PbO (in slag)-ICO (10) the lead content of the slag being proportional to the CO /CO ratio. 'Usually, with zinc-rich charges, this second limitation is the primary controlling condition.

With a limitation thus set to the permissible lowest value of the CO/CO ratio, there is a corresponding limit set to the amount of slag that can be melted for unit weight of carbon consumed. Essential features of the operations in the slagging zone are that suflicient heat must be generated to melt the slag, subject to the condition that the gas produced must contain a certain fraction of the carbon as carbon monoxide and that certain endothermic reactions must occur in proportion to the amount of carbon burnt. This means that with a slag of a given zinc content, the amount of slag to be melted dictates the carbon requirements, which so far as the conditions in the slagging zone are concerned does not depend on the amount of lead to be reduced.

The gases generated in the sla-gging zone enter the middle zone at the same temperature as the charge and in equilibrium with respect to Reactions 7 and 9. Disturbance of this thermal and chemical equilibrium between gas and charge tends to be brought about in two ways. Firstly, there is some heat loss through the furnace walls; secondly, and usually more important, although the reaction of carbon with carbon dioxide (to produce carbon monoxide according to Equation 6) is slow compared with diffusion-controlled reactions, it proceeds to a considerable extent at the temperature prevailing in the mid dle zone of the furnace. As this endothermic reaction takes place, the temperature becomes lower as the gases proceed up the furnace. As the temperature falls, the equilibrium constants of Reactions 7 and 9 become smaller in value, so that, to maintain the chemical equilibrium in these reactions, lead sulphide and zinc vapour are removed by the reverse reactions:

PbS (gas) +ZnO +CO =Pb+ZnS+CO (11) and Zn (gas) +CO =ZnO+CO (12) Simultaneously removal of lead sulphide and zinc can, of course, take place by reversal of Reaction 8:

With falling temperature, some of the lead vapour also condenses. As, proceeding up the furnace, the temperature falls (substantial equality of temperature being main tained between charge and gases) Reaction 6 becomes slower. A temperature of about 1000 C. for both gases and charge is attained near the top of the middle zone of the furnace, that is, not far below the level at which reduction of lead oxide has been completed. By the time this level has been reached, nearly all of the lead sulphide and most of the lead vapour have been removed from the gas, but much of the zinc vapour originally generated in the slagging zone remains, the zinc concentration re maining being determined by the equilibrium constant of Reaction 7 at the temperature prevailing, which is about 1000 C.

As a result of passing up through this middle zone of the furnace the carbon-monoxide content of the gases is considerably increased, and this is the important metallurgical aspect of the processes occurring in this middle zone, particularly with zinc-rich charges. If the slag contains much zinc oxide, the large amount of heat absorbed in forming zinc vapour by Reaction 7 precludes the possibility of generating, in the slagging zone, a gas containing a high CO/CO ratio and it is estimated that the gases leaving the slagging zone may contain as much as four times as much carbon dioxide as carbon monoxide. As shown by the discussion of Equation 10, the lead content of slag is proportional to the CO /CO ratio in the slagging zone, so this explains why higher lead contents of slag are obtained with zinc-rich than with zinc-free charges; typically a lead blast furnace with zincrich charges operates with the lead content of slag 1.5 weight percent or higher, whereas lower lead contents can be obtained with zinc-free charges. Further, with zinc-rich charges, the amount of carbon monoxide generated in the slagging zone would usually be insuflicient for reducing the lead oxide higher up in the furnace. By the time that the gases have passed up through the middle zone of the furnace, more carbon monoxide has been generated by Reactions 6 and 12 so that the contents of carbon monoxide and carbon dioxide have become approximately equal: a typical gas composition at the top of the middle zone just below the lead-reduction zone, is estimated to be 3.3% Zn, 14% CO, 13% CO balance mostly nitrogen.

These gases entering the lead-reduction zone contain, besides carbon monoxide, which can reduce lead oxide by Reaction 1, a smaller but appreciable amount of zinc vapour, which can reduce lead oxide :by the reaction:

The reduction of lead oxide by carbon monoxide (Reaction 1) is exothermic, and by zinc vapour (Reaction 14) is highly exothermic. The downcoming charge becomes heated by the heat given out in these two reactions. The greater the ratio of zinc vapour to carbon monoxide present in the gas, the greater becomes the amount of heat generated for a given amount of lead oxide reduced. The rise of temperature in the charge for unit amount of heat generated is inversely proportional to its heat capacity, which is the sum of the heat capacities of the lead oxide, coke, and slag-forming materials present. Thus, with a constant ratio of zinc vapour to carbon monoxide in the gas, the rise of temperature due to those exothermic reactions is greater the higher the ratio of lead oxide to slag-forming materials present.

The temperature rise in the charge in the lead-reduction zone cannot simply be calculated from the ratio of the heat generated in Reactions 1 and 14 to the heat capacity of the charge. Firstly, as mentioned before, some endothermic reactions producing sulphur dioxide take place, as shown by Equations 2 and 3. Secondly, there is also convective heat transfer between gases and charge. At temperatures up to about 400 C. the reduction of lead oxide is relatively slow, and the charge is heated up to at least this temperature almost entirely by convection heat transfer from the hotter gases. Even when the charge temperature has risen to 600 C. and the reduction of lead oxide by carbon monoxide is an intrinsically fast reaction, its actual rate is governed by the rate of diffusion of carbon monoxide from gas to charge surface and of carbon dioxide from charge surface to gas. There is a proportionality between the rates of such diffusion-controlled reactions and the rate of convective heat-transfer, with the consequence that, so long as there is any considerable difference of temperature between gas and charge (say 200 C.) the rate of heat transfer 'by convection is comparable with the heat generated at the solid surface by Reaction 1, provided that there is a considerable concentration (say by volume) of carbon monoxide in the gas. This means that if in, say, the part of the lead-reduction zone where the temperature of the charge is between 500 C. and 600 C., the volume concentration of carbon monoxide is only a small fraction of 10%, only very little lead oxide can be reduced by carbon monoxide in this region. As a consequence, although the equilibrium in Reaction 1 permits almost complete conversion of carbon monoxide to carbon dioxide, the competition between heat transfer and mass transfer in the lead-reduction zone prevents such complete utilization of the carbon monoxide; it is estimated that this condition means that, of the oxides of carbon in the gases leaving the top of the lead-reduction zone, at least must be present as carbon monoxide and not more than 85% as carbon dioxide. Taking the previously mentioned typical composition of the gas entering the lead-reduction zone (3.3% Zn, 14% CO, 13% CO this means that, per gram-atom of carbon consumed, there are 0.48 mole of CO and 0.12 mole of Zn, making a total of 0.60 mole, 85% utilization of which would enable 0.85 0.60=0.51 mole of lead oxide to be reduced for one gram-atom of carbon, which corresponds to the weight of lead reduced from its oxide being 8.8 times the weight of carbon consumed; this figure of 8.8 would be reduced if some other substances, such as sulphate (CaSO if present in the charge, for example) have to be reduced. In the conditions under which a lead blast furnace is normally run, other considerations prevent the attainment of this utilization of the reducing agents in the gases.

Even when the gas finally leaving the charge contains an appreciable amount of carbon monoxide, the charge, particularly in the upper regions of the lead-reduction zone, receives comparable amounts of heat from convective heat transfer from the gas and from the heat generated by reduction of lead oxide by carbon monoxide and by zinc. With a given ratio of carbon monoxide to zinc in the gas entering the lead-reduction zone, the greater the ratio of lead oxide to slag-forming materials in the charge, the higher is the temperature attained by the charge before all the lead oxide has been reduced.

If the ratio of lead oxide to slag-forming materials is low enough, all the lead oxide will be reduced before its melting-point is reached-the effective melting point will be somewhat below 880 C., the melting point of pure lead oxide. While such conditions, in which all the reduction is effected from solid charge, represent the most favourable conditions for attaining smooth operation, no serious operating troubles should arise if some small fraction of the lead oxide has to be reduced from the fused state, as happens if the ratio of lead oxide to slag-forming materials is increased somewhat, since this merely means that a small amount of molten lead oxide has to be reduced to molten lead. Before the temperature of the charge has risen much further, however, other troubles may arise. Although lower down in the furnace, where all the lead oxide has been reduced, lead sulphide as such does not exist in the charge, so long as lead oxide is present lead sulphide is stable, and therefore, once a temperature has been attained at which lead sulphide has a considerable vapour pressure, volatilization of lead sulphide takes place in considerable amounts. As the temperature of the charge increases, it begins to approach that of the gas, and if it reaches about 1000 C., becomes equal to that of the gas. If there is still lead oxide present at 1000 C., the charge becomes still further heated owing to the heat generated by the reduction reactions, and a point may be reached at which, not only the lead oxide, but also some of the slag-forming components start to melt.

Once all the lead oxide has been reduced, the rise in temperature of charge ceases, and, since the temperature of the gas here is lower than that of the charge, the convective heat transfer causes the temperature of the charge to fall again as it descends further in the furnace. This cooling causes the molten portion of the charge to resolidify and this tends to cement the charge together; this is regarded as the essential cause of the tendency for the charge to form bridges across the furnace and to hang-up in the furnace when an attempt is made to smelt lead-rich charges.

It is estimated that, under typical conditions, these operating troubles arise unless the weight of slag-forming materials in the charge is at least 1.1 times the weight of oxidized lead.

The physico-chemical picture that has been presented of the lead blast furnace must be to a certain extent idealized, but it is believed that it represents the important controlling features of operation. It leads to the idea that the furnace has three zones; in the slagging zone, the main problem is that of heat generation, in the middle zone the main problem is that of generating reducing agent, and in the top lead-reduction zone the problem is that of heat dissipation. The conditions obtaining in these three zones are necessarily interdependent. Accurate estimates of limitation in performance in any one respect cannot therefore be given without taking into account all aspects of the performance. Nevertheless, as already indicated, approximate figures can be given for limitations on separate aspects of performance.

In the slagging zone, sufiicient heat has to be generated to melt the slag, this heat being generated under such conditions that the gases produced are sufficiently reducing to prevent much lead oxide being dissolved in the slag. With zinc-rich charges, this limitation means that the weight of slag melted must not be greater than about 7.0 times the weight of carbon burnt. The other two limitations have already been given. The available reducing agent limits the weight of lead charged as oxide to 8.8 times the weight of carbon consumed. The problem of heat dissipation in the lead-reduction zone means that the weight of slag produced must be at least 1.1 times the weight of oxidized lead in the charge.

It will be noticed that the first and third of these conditions imply that the weight of oxidized lead charged must not be greater than 7.0/l.1=6.4 times the weight of carbonconsumed. The conclusion is therefore then reached that the amount of available reducing agent (which would permit oxidized lead up to 8.8 times the weight of carbon burnt) is not a primary limitation on the performance. The primary limitations are that the slag weight must be at least 1.1 times the weight of oxidized lead charged and that the carbon consumed must be at least one-seventh of the slag weight.

This explains why, with high-grade lead concentrates, it is necessary to add inert materials, either during the sinter-roasting or direct to the blast furnace, in order to obtain smooth operating conditions. The sintered material often contains part of the lead as metal; this lead metal does not contribute any heat of reduction, so that it is only the oxidized lead that has to be taken into con- 9 sideration in determining the amount of inert materials that must be incorporated.

(B) Operation of a lead blast furnace according to the invention As explained under (A) we assume that in the slagging Zone of the furnace the gases generated are at a temperature approximately equal to the melting point of the slag and that the ratio of carbon monoxide to carbon dioxide adjusts itself so as to satisfy the heat balance, i.e. to provide heat to melt the slag and to satisfy the endothermic reactions.

The carbon monoxide to carbon dioxide ratio must also not fall below a minimum value usually set as previously mentioned by Equation 10 for a particular maximum figure of lead in slag since there is at least some approach to equilibrium in this reaction.

Also, of course, the carbon monoxide and carbon dioxide present must be sufiicient so that after passing through the middle zone, where more carbon monoxide is formed, there will be sufficient carbon monoxide to reduce the lead oxide in the top zone, taking into account the inefficiency inherent in the top zone which has been described.

In general, if all of the heat for slagging is supplied by combustion of carbon in the slagging zone, and if the carbon monoxide to carbon dioxide ratio satisfies the minimum conditions, the amount of carbon monoxide formed will be in excess of that required according to the previous paragraph.

There is thus an inefficiency, contrary to what has previously been assumed, since at least part of the carbon which is being burnt to a mixture of carbon monoxide and carbon dioxide in the requisite ratio could be more efficiently used to provide heat by being burnt to carbon dioxide and the heat evolved used in an eflicient preheater to preheat the blast.

Thus the total carbon consumed per unit of slag melted could be reduced while keeping the carbon monoxide and carbon dioxide ratio in the slagging zone constant and therefore the amount of lead in the slag constant.

If the total carbon consumed is kept constant then the carbon monoxide to carbon dioxide ratio could be increased and hence lead in slag, which is dependent on this ratio according to Equation 10, will be reduced.

A combination of these two etfects could also be produced.

When steam is introduced into the slagging zone, some of it is reduced to hydrogen:

Some of the sensible heat in the preheated air is utilized for liberating hydrogen from the water vapour, but, under the conditions normally prevailing, most of the water vapour leaves the slagging zone as such.

In the middle zone of the furnace, water vapour reacts with coke according to Equation while simultaneously carbon dioxide reacts with the coke to produce carbon monoxide according to Equation 6. The rate of reaction of coke with water vapour is considerably greater than with carbon dioxide. Therefore, as the gases proceed up the furnace, Reaction 15 takes place more rapidly than Reaction 6, so that, by the time the gases have passed through the middle zone of the furnace and reached a level just below the lead-reduction zone, a considerable amount of hydrogen has been produced and the temperature has been reduced to a lower value than is attained in the absence of steam. Because of the change with temperature of the equilibrium constant of Reaction 7 the lower temperature means less zinc vapour in the gas.

As an example, it is estimated that if steam is added to the air blast until it contains (by volume) 18.7% Water vapour, and this air is preheated to 500 C. before being introduced into the furnace tuyeres, the temperature will fall to 930 C. at the top of the middle zone 10 of the furnace (that is, just below the lead reduction zone)this compares with 1000 C. for dry air blast and the gas produced here is calculated to contain, by volume, 1.3% Zn, 14% CO, 12% CO 7.2% H and 9.2% H 0.

When the gases enter the lead-reduction zone less heat is generated for unit amount of lead reduced. Firstly, the lower content of zinc vapour means that less heat is evolved by its highly exothermic reaction Equation 14. Secondly, the reduction of lead oxide by hydrogen:

evolves considerably less heat than the reduction by carbon monoxide (Equation 1).

Further, the diffusion-controlled reduction with hydrogen (Equation 16) proceeds considerably more rapidly than does that with carbon monoxide, so that hydrogen can be almost completely utilized as a reducing agent, whereas carbon monoxide cannot. The total result of all effects is that, because less heat is generated in the lead-reduction zone at the top of the furnace when steam is introduced with the air blast, the amount of inert material that has to be introduced in the charge to .absorb this heat is considerably decreased, with a consequent decrease in the heat demand imposed on the slagging zone.

In the light of the foregoing discussion and examples, the following three inequalities can be used to represent algebraically the conditions under which a lead blast furnace can be operated.

(1) Slag/lead Aa(percent H 0). (2) Lead/ carbon B+b(percent H 0). (3) Slag/carbon C+c(blast C.).

c(percent H O).

Slag and carbon refer to the weights of slag run off and carbon charged in unit period, and lead the weight, in the same unit period, of lead charged in oxidized form, that is, of the weight of metallic lead run off less the weight of lead already present in the metallic form in the charge. Blast C. refers to the temperature of the blast, and percent H O to the volume percent H O in the blast. The letter symbols (A, B, C, a, b, c and 0) represent parameters that are constant for particular circumstances, but their values depend somewhat on the properties of the carbonaceous fuel-usually cokeused (particularly on its reactivity towards carbon dioxide and water vapour), on charge composition (for example on the amount of zinc present) and on operating conditions (these vary somewhat, for example, according to how low a lead content of slag it is desired to attain). When the blast consists of atmospheric air and water vapour, the following are the ranges of values of these parameters for the usual range of conditions encountered. For convenience of subsequent discussion, a typical value has been selected within the specified range for each parameter:

A=1.01.2, typical value 1.1 B=8.010.0, typical value 8.8 C=6.07.4, typical value 7.0 a=0.0200.030, typical value 0.025 b=O.18-0.27, typical value 0.22 c=0.0140.016, typical value 0.015 c'=0.250.40, typical value 0.33

With the appropriate values for these parameters inserted, the above three inequalities specify operable conditions. If the slag/lead ratio in the charge is less than A, there must be added at least sufficient steam to satisfy inequality 1. With any value chosen for percent H O, the highest permissible value for the lead/carbon ratio is then fixed, and any value lower than this can be chosen. Once the lead/ carbon ratio is fixed, the slag-carbon ratio is fixed also (for a particular charge) and the blast preheat must then be fixed so that inequality 3 is satisfied.

(1) Slag/lead 1.1-0.025 (percent H (2) Lead/carbon 8.8+0.22 (percent H 0). (3) Slag/carbon 7.0+0.015 (blast C.).

0.33 (percent H O).

With these typical values for the parameters, it is possible to illustrate graphically the scope of the invention as applied to typical furnace conditions. For completeness of presentation of the implications of these inequalities, the graphical presentation is extended to regions of high slag/ lead ratios that are outside the claimed scope of the present invention.

FIGURE 1 is a graph showing preheat in degree centigrade plotted against slag to lead produced ratio for a typical furnace. On the same graph are also shown the percentage water vapour additions for operating the furnace according to the invention, below a slag a lead ratio of 1.1: 1, without recycling of the slag.

FIGURE 2 is a graph showing the lead produced to carbon consumed ratio for various slag to lead produced ratios in respect of the prior art and in accordance with the invention.

FIGURE 3 is a graph showing the increase in the ratio of lead produced to carbon consumed effected according to the invention by each 100 C. of blast preheat.

FIGURE 4 is a graph showing the additional preheat over and above the line DEF of FIGURE 1 for each 1% of additional water vapour introduced.

FIGURE 5 is a graph showing the increase in lead produced per ton of carbon consumed in preheaters and boilers plotted against the slag to lead produced ratios.

With no steam added to the blast, the three inequalities become:

(l) Slag/lead 1.1.

(2) Lead/carbon 8.8.

(3) Slag/carbon M 7.0+0.015 (blast C.).

In order to obtain the best fuel economy, inequalities 2 and 3 must interpreted as equations.

Pb/C=8.8 and Slag/C=7.0+0.0l5 C. preheat) so that 8.8 (slag/Pb=7.0+0.015 C. preheat) and the required preheat is:

8.8 (slag/Pb) 7.0

This is represented by the line EF of FIGURE 1, for slag/ Pb ratios greater than 1.1.

The line JK of FIGURE 2 shows that, with the slag/ lead ratio greater than 1.1 and no steam in the blast, the Pb/C ratio is maintained at 8.8 irrespective of slag/lead ratio. According to the prior art, the slag/C ratio is restricted to 7.0, and therefore the lead/C ratio decreases with the slag/ C ratio as shown by the line PQ of FIGURE 2.

So long as no steam is added to the blast, a limitation in lead smelting is that ratio of slag to oxidized lead in the charge must not be allowed to fall below a certain figure, about 1.1, as explained previously. This restriction is relaxed when steam is incorporated in the blast, the extent of this relaxation being given by inequality 1:

Slag/Pb 1.10.02S (percent H O) in blast),

Percent H O in blast 4440 slag/ Pb This minimum steam content of blast is shown by the line AB in FIGURE 1.

If the steam content of the blast is set at this minimum figure, the maximum weight of oxidized lead that can be reduced for unit weight of carbon is given by inequality 2 written as an equation:

Pb/C=8.8+0.22 (percent H O in blast) with Percent H O in blast=44-40 slag/ Pb this becomes Pb/C=18.48-8.8 slag/Pb This is shown in FIGURE 2 by the line I].

To attain these lead carbon ratios it is necessary to use the appropriate amount of preheat, according to the inequality 3, which, to attain the best fuel economy, that is, the highest possible slag/ carbon ratio, can be written as an equation:

Slag/carbon=7.0

+0.015 (blast C.)0.33 (percent H O).

which can be rewritten:

0.015 (blast C.)=slag/carbon 7.0+0.33 (percent H O) Dividing both sides of this equation by 0.015 gives:

Blast C=67 (slag/carbon7.0) +22 (percent H O) The required prehcats are shown by line DE in FIG- URE 1.

The line LMN in FIGURE 2 also shows the ratio of lead reduced to total carbon consumed (in furnace, preheater, and boiler) on the assumption that for each ton of furnace carbon used, 0.04 ton of carbon is consumed for each C. by which the blast is preheated and 0.006 ton of carbon is consumed for each 1% by volume of steam introduced into the air blast.

If the slag/Pb ratio is less than 1.1, it is necessary to add at least the amount of steam indicated by the line AB of FIGURE 1, in order to avoid hot-top conditions. The prior art for slag/Pb ratio less than 1.1 is to dilute with inert materials to bring the slag/ lead ratio up to 1.1; without preheated air this means that the slag/carbon ratio stays constant at 7.0 and the lead/carbon ratio at 7.0/1.1: 6.36 as shown by the line OP (FIGURE 2). Compared with this if, for example, at a slag/Pb ratio of 0.6, and with the necessary 20% H O in the blast, the heat balance gives:

Slag/C=7.0+0.015 C. preheat) 0.33 (percent H O in blast) =0.4+0.015 C. preheat) Pb/C: (slag/C)0.6:0.67+0.025 C. preheat) With no preheat the Pb/C ratio would be only 0.67. To bring it up to the prior art level of 6.36 requires In general to equal the ratio of lead reduced to carbon consumed as attained in the prior art, the blast preheat must be at least at high as shown by the line GB (FIG- URE 1). To bring the Pb/total C (total C is C consumed in furnace, preheater and boiler) ratio down to that of the prior art, the preheat must be at least up to the line HB (FIGURE 1).

With the steam content of the blast set at its minimum permissible value along the line ABC (FIGURE 1) (no steam added along BC), GBC represents the minimum temperature of blast that must be used to equal the ratio of reduced lead to furnace carbon (Pb/ C) attained in the prior art, as shown by the line OPQ of FIGURE 2. Increasing the preheat above that shown by the line GBC up to the line DEF (FIGURE 1) increases the Pb/furnace C ratio up to that shown by the line IJK (FIGURE 2). Within this range, between the lines GBC and DEF (FIGURE 1), the increase of lead/carbon ratio (that is, the increase in the weight of lead reduced for unit weight of carbon burnt in the furnace) for each extra 100 C. of blast preheat is as shown by the line RS (FIGURE 3).

With the moisture content of the blast fixed as shown by the line ABC (FIGURE 1), the line RS (FIGURE 3) shows the increase in the lead/carbon ratio (that is, the increase in the weight of lead reduced for unit weight of carbon burnt in the furnace) for each additional 100 C. of blast preheat up to the line DEF (FIGURE 1). The line TU (FIGURE 3) shows the increase in the lead/ carbon ratio for each additional 100 C. of blast preheat above the line DEF.

With slag/ Pb ratio greater thaan 1.1, continuous benefit in respect of increased furnace output (Pb/furnace C) and fuel economy (Pb/total C) is achieved by the use of increased preheat, without steam, up to the limit of the line EF drawn in FIGURE 1 and therefore, although steam could be used with preheat below the level EF with advantage over the prior art, it would not give as much improvement as can be obtained by preheat alone. Compared with the prior art, using no preheat, represented by the line BC (FIGURE 1), improved performance is attained anywhere in the temperature region BEFC, the higher the temperature (up to the line EF) the higher the furnace output and the better the fuel economy.

If more preheat is to be used than the line DE of FIGURE 1, more water vapour also must be used and, if additional preheat is required above the line EF, water vapour must be introduced into the blast. The additional preheat that has to be used with each 1% steam above the line DEF (FIGURE 1) is shown by FIGURE 4.

When extra preheat (FIGURE 1) is used above the line DEF, and extra steam above the line ABC is also used, much of the extra preheat is used for providing hydrogen as extra reducing agent. Consequently the increase of lead/ carbon ratio for an extra 100" C. of preheat above the line DEF (FIGURE 1), which is shown by the line TU in FIGURE 3, is considerably less than in the preheat region below the line DEF in FIGURE 1.

The curves RS and TU (FIGURE 3) can be represented in terms of extra lead produced per ton of carbon consumed in preheaters and boilers. This is done in FIGURE 5.

In FIGURE 5 VW represents increased weight of lead produced for unit weight of carbon burnt in preheaters and boilers up to the level of the line DEF in FIGURE 1. XY represents the increased weight of lead produced for unit weight of carbon burnt in preheaters and boilers when the air preheat is increased above the line DEF and extra steam has to be introduced above the level ABC (FIGURE 1). On comparing this VW curve with the LMN curves of FIGURE 2, reproduced also in FIG- URE 5, it can be seen that in increasing the preheat up to the level DEF (FIGURE 1) the marginal yield of lead per unit of carbon burnt in preheaters is vastly greater than the total yield of lead per unit of carbon, so that increased use of preheat up to the DEF level is highly beneficial on furnace performance. Curve XY (FIGURE 5) is much lower than VW, and while always higher than LMN, the marginal yield of lead per ton of extra carbon is only 1.11.3 times the total yield obtained along the line DEF of FIGURE 1, whereas the curve VW (FIG- URE 5) gave marginal yields 3-8 times that of LMN. While normally it is therefore always advantageous to increase preheats up to the line DEF of FIGURE 1, whether it is advantageous to increase the preheat further may depend on circumstances. Under some circumstances the curve DEF, with the steam concentration of blast as given by the line ABC, will be the preferred region. Under other circumstances it will be preferable to increase the air preheat up to the capacity of the preheater available, with a corresponding increase of the steam addition above the line ABC.

When the amount of steam added is only the minimum permissible amount, the scope of the invention is that given with the three inequalities written as equations, which give, for any slag/lead ratio, the required lead/ carbon ratio, as well as the temperature and steam content of blast. For typical furnace conditions, FIGURES 1 and 2 give these results, which would vary somewhat with variation of furnace conditions and consequent alteration, within the specified ranges, of the values of the parameters in the three inequalities. Accordingly, the invention comprises the method of operating a lead blastfurnace in which materials containing lead oxide and carbon are charged at the top of the furnace, molten lead, containing readily reducible metals such as silver, is tapped from the bottom of the furnace, and a molten slag, of low lead content, but containing the less readily reducible oxides present in the charge, is also removed from the bottom of the furnace, characterized in that the weight of lead charged to the furnace is more than the weight of slag tapped from the furnace and a preheated blast consisting of air and water vapour is supplied to the bottom of the furnace, the volume concentration of water vapour in the blast being controlled, according to the slag/lead ratio, approximately as shown by the line AB of FIG- URE 1 and the blast temperature being approximately as shown by the curve DE of FIGURE 1.

The invention as thus defined rests on three equations, which, owing to possible variations of the parameters, have to be regarded as approximations:

(1) Percent H O in blast=4440 (slag/ lead), which may be written Percent H O=4.0+40 (l.0slag/lead) (2) Lead/carbon=8.8+0.22 (percent H 0) (3) Blast C.=67 (slag/carbon7.0)+22 (percent H O) The invention comprises the method of operating a lead blast-furnace in which materials containing lead oxide and carbon are charged at the top of the furnace, irnolten lead, containing readily reducible metals, such as silver, is tapped from the bottom of the furnace, and a molten slag, of low lead content but containing the less readily re ducible oxides present in the charge, is also removed from the bottom of the furnace, characterized in that the weight of lead charged to the furnace is more than the Weight of slag tapped from the furnace and a preheated blast consisting of air and water vapour is supplied to the bottom of the furnace, and that the volume concentration of water vapour in the blast exceeds 4.0 by approximately 40 times the amount by which the ratio of slag to oxidized lead in the charge is less than 1.0, the weight ratio of oxidized lead to carbon in the charge exceeds 8.8 by approximately 0.212 times the volume percentage of water vapour in the blast, and the blast temperature, in C., is approximately equal to the sum of 22 times the volume percentage of water vapour in the blast and -67 times the excess of the slag/ carbon ratio over 7.0.

Although, for the best fuel economy, inequalities 2 and 3 can be interpreted as equations, the same is not always true for inequality 1, because, as already explained, it is sometimes advantageous to increase the amount of water vapour above the minimum value necessary to prevent hot conditions near the furnace top. To define the scope of the invention in relation to this inequality, it may be noted from FIGURE 1 that the lead/carbon ratio always exceeds 8.8 and that the blast preheat is always at least 200 C. Accordingly, the invention consists in the method of operating a lead blast-furnace in which the Weight ratio of lead to carbon in the charge exceeds 8.8, the weight of lead charged to the furnace is more than the weight of slag tapped from the furnace, and a [mixture of air and water vapour, preheated to at least 200 C., is sup plied to the bottom of the furnace, the percentage volume concentration of water vapour in the blast exceeding 4.0 by at least 40 times the amount by which the slag/lead ratio in the charge is less than 1.0.

For further defining the scope of the invention, the

1 lower and upper values of the parameters can be inserted in equality 2, to give -Lead/carbon 8.04-0.18 (percent H 0) and Lead/carbon l0.0i-O.27 (percent H O) To obtain the most economic .use of carbon, these inequalities should be replaced by equations. That is to say, the permissible maximum lead/carbon ratio lies between 8.0+0.18 (percent H 0) and 10.0+0.27 (percent H O).

The invention therefore further consists in a method of operating a lead blastaf-urnace in which the weight of lead charged to the furnace is more than the weight of slag tapped from the furnace, and the air supplied to the bottom of the furnace contains at least 4% by volume of water vapour and is preheated to at least 200 C., the amount of Water vapour in the blast being sufiicient to prevent an excessively high temperature being generated near the furnace top, the weight ratio of oxidized lead to carbon in the charge exceeding 8.0 by at least 0.18 times the volume percentage of water vapour in the blast but not exceeding 10.0 by more than 0.27 times the volume percentage of water vapour in the blast.

The degree of preheat required in the blast is given by inequality 3 which, to obtain the most economic use of preheat, can be written as an equation 0 (blast C.)=slag/carbon-C+c' (percent H O) Putting C=7.4, 0:0.016, c=0.25, (blast C.) :62 (slag/ carbon-7.4) 15 (percent H O) Putting C=6.0, c=0.0l4, c=0.40, (blast C.)=71 (slag/ carbon6.0)+29 (percent H O).

The required blast temperature, in C., is therefore at least equal to the sum of 15 times the volume percentage of water vapour in the blast, and 62 times the excess of the slag/carbon ratio over 7.4 but is not greater than the sum of 29 times the volume of Water vapour in the blast and 71 times the excess of the slag/carbon ratio over 6.0.

As an example of the invention, the case will be considered of a sulphide concentrate containing 68.0% Pb, 7.5% Zn, 4.2% Fe, 1.7% SiO 0.4% CaO and 17.3% S. To produce suflicient sinter for a days charge to the blast furnace, 750 tons of this concentrate is sinter-roasted with fiuxes containing 45 tons OaO, 52 tons Si0 and iron equivalent to 50 tons FeO. The sinter contains 510 tons lead, (51 tons) in the metallic state, the remaining 459 tons in oxidized form. This sinter is charged to the furnace during a .days operation with 50 tons of coke; the coke contains 20% ash, this ash containing 46% SiO 8% 'FeO (iron calculated as FeO) and 2% CaO. The blast used contains 14% by volume of water vapour and it is preheated to 450 C. The Weight of sla'g run off from the furnace is 340 tons, this slag containing 20.8% ZnO, 28.7% FeO, 20.4% Si0 and 14.2% CaO.

According to the prior art, such a concentrate would be sintered with the required amounts of fluxes, together with sufficient recirculated blast-furnace slag to bring the total weight of slag at least up to the weight of oxidized lead. With 750 tons of concentrate, this would require the addition of about 120 tons of slag, so that the total weight of slag produced would be 460 tons. With cold air used as blast, there would be required about 68 tons of carbon, compared with the 40 tons carbon (50 tons coke) used according to the invention. That is to say, the amount of furnace coke required for unit weight of lead produced is 1.7 times as great in the prior art as according to the in- 'vention. Alternatively regarded, for a furnace of given coke-burning capacity, the weight of lead that can be produced is 1.7 times as great according to the invention as according to the prior art.

It is estimated that, for each ton of furnace carbon used, 0.04 ton of carbon is consumed for each 100 C., by which the blast is preheated and 0.006 ton of carbon is consumed for each 1% by volume of steam introduced into the air blast.

With blast containing 14% of water vapour and preheated to 450 C., the carbon required for steam-raising and preheating would be ton carbon per ton of furnace carbon. That is, for 40 tons of furnace carbon, there would also be required 10.6 tons of carbon for preheating and steam raising. The total carbon consumption, 50.6 tons, compares with 68 tons required according to the prior art.

I claim:

1. In the method of operating a lead blast-furnace in which materials containing lead in oxidized form and carbon are charged at the top of the furnace, molten lead is tapped from the bottom of the furnace, and a molten slag, of low lead content but containing any oxides present in the charge which are less reducible than lead oxide, is also removed from the bottom of the furnace, the improvement in combination therewith which comprises charging to the furnace a weight of lead in oxidized form greater than the weight of slag tapped from the furnace, the Weight ratio of lead in oxidized form to carbon in the charge exceeding 8.8; supplying an air blast at the bottom of the furnace containing at least 4% by volume of water vapour; preheating the water vapour to at least 200 C.; and controlling the amount of Water vapour in the blast to prevent an excessively high temperature being generated near the furnace top to inhibit the tendency of the charge to form bridges of the charge across the furnace with consequent uneven descent and hanging-up of the descending charge and for avoiding the customary necessity of diluting the charge with inert material to keep the top portion of the charge from getting too hot.

2. In the method of operating a lead blast-furnace in which materials containing lead in oxidized form and carbon are charged at the top of the furnace, molten lead is tapped from the bottom of the furnace, and a molten slag, of low lead content but containing any oxides present in the charge which are less reducible than the lead oxide, is also removed from the bottom of the furnace, the improvement in combination therewith which comprises charging to the furnace a weight of lead in oxidized form greater than the weight of the slag tapped from the furnace; supplying a preheated blast consisting of air and water vapour to the bottom of the furnace; controlling the volume concentration of water vapour in the blast according to the ratio of slag run off from the furnace to lead charged in oxidized form to the furnace, approximately as shown by the line AB of FIGURE 1; and controlling the temperature of the blast approximately as shown by the curve DE of FIGURE 1, to prevent an excessively high temperature being generated near the furnace top to inhibit the tendency 'of the charge to form bridges of the charge across the furnace with consequent uneven descent and hanging-up of the descending charge and for avoiding the customary necessity of diluting the charge with inert material to keep the top portion of the charge from getting too hot.

3. A method as claimed in claim 2, in which the ratio of lead in oxidized form to carbon in the charge is approximately as shown by the line I] of FIGURE 2.

4. In the method of operating a lead blast-furnace in which materials containing lead in oxidized form and carbon are charged at the top of the furnace, molten lead is tapped from the bottom of the furnace, and a molten slag, of low lead content but containing any oxides present in the charge which are less reducible than lead oxide, is also removed from the bottom of the furnace, the improvement in combination therewith which comprises charging a Weight of lead to the furnace in oxidized form greater than the weight of slag tapped from the furnace; supplying a preheated blast consisting of air and added water vapour to the bottom of the furnace; controlling the volume concentration of water vapour in he bla t [to exceed 4.0 by approximately 40 times the amount by which the ratio of slag to lead in oxidized form in the charge is less than 1.0; controlling the weight ratio of lead in oxidized form to carbon in the charge to exceed 8.8 by approximately 0.22 times the volume percentage of water vapour in the blast; and controlling the blast temperature in C., to make it approximately equal to the sum of 22 times the volume percentage of water vapour in the blast and 67 times the excess of the slag/ carbon ratio over 7.0, to prevent an excessively high temperature being generated near the furnace top to inhibit the tendency of the charge to form bridges of the charge across the furnace with consequent uneven descent and hanging-up of the descending charge and for avoiding the customary necessity of diluting the charge with inert material to keep the top portion of the charge from getting too hot.

5. In the method of operating a lead blast-furnace in which materials containing lead in oxidized form and carbon are charged at the top of the furnace, molten lead is tapped from the bottom of the furnace, and a molten slag, of low lead content but containing any oxides present in the charge which are less reducible than the lead oxide, is also removed from the bottom of the furnace, the improvement in combination therewith which comprises charging to the furnace a weight ratio of lead in oxidized form to carbon that exceeds 8.8; controlling the weight of lead in oxidized form charged to the furnace to more than the weight of slag tapped from the furnace; preheating a mixture of air and Water vapour to at least 200 C.; supplying the preheated air and water vapour mixture to the bottom of the furnace; controlling the percentage volume concentration of Water vapour in the blast to exceed 4.0 by at least 40 times the amount by which the ratio of slag run off from the furnace to lead charged in oxidized form to the furnace in the charge is less than 1.0, to prevent an excessively high temperature being generated near the furnace top to inhibit the tendency of the charge to form bridges of the charge across the furnace with consequent uneven descent and hanging-up of the descending charge and for avoiding the customary necessity of diluting the charge with inert material to keep the top portion of the charge from getting too hot.

6. In the method of operating a lead blast-furnace in which materials containing lead in oxidized form and carbon are charged at the top of the furnace, molten lead is tapped from the bottom of the furnace, and a molten slag, of low lead content but containing any oxides present in the charge which are less reducible than lead oxide, is also removed from the bottom of the furnace, the improvement in combination therewith which comprises charging a weight of lead in oxidized form to the furnace greater than the Weight of slag tapped from the furnace; preheating the air supplied to the bottom of the furnace containing at least 4% by volume of water vapour to a temperature of at least 200 C.; controlling the weight ratio of oxidized lead to carbon in the charge to exceed 8.0 by at least 0.18 times the volume percentage of water vapour in the blast but not exceeding 10.0 by more than 0.27 times the volume percentage of water vapour in the blast and employing an amount of water vapour in the blast sufliciently high to prevent an excessively high temperature being generated near the furnace top to inhibit the tendency of the charge to form bridges of the charge across the furnace with consequent uneven descent and hanging-up of the descending charge and for avoiding the customary necessity of diluting the charge with inert material to keep the top portion of the charge from getting too hot.

7. A method as claimed in claim 6, preheating the air to a temperature in C. at least equal to the sum of 15 times the volume percentage of water vapour in the blast and 62 times the excess of the slag/carbon ratio over 7.4 but is not greater than the sum of 29 times the volume of water vapour in the blast and 71 times the excess of the slag/ carbon ratio over 6.0.

8. In the method of operating a lead blast-furnace in which a charge containing lead oxide and carbon is fed in at the top of the furnace, molten lead is tapped from the bottom of the furnace, and a molten slag of low lead content but containing any oxides present in the charge which are less reducible than the lead oxide, is also removed from the bottom of the furnace, the improvement in combination therewith which comprises introducing into the furnace a charge having a ratio of slag to oxidized lead less than 1.0; preheating a blast of a mixture of air and water vapour to a temperature range of 200 C. to 550 C.; controlling the amount of water vapour in the blast to contain a range of 428% by volume; reducing and controlling the high temperature that develops at the top of the furnace when the normal blast of cool air is used with such a charge by regulating the minimum amount of water vapour in the blast to be proportional to the amount by which the ratio of slag produced to lead as oxide in the charge falls below about 1.1; regulating the highest permissible ratio of lead to carbon in the charge to be dependent on the amount of water vapour in the blast; and controlling the minimum amount of preheating of the blast to depend on the amount of water vapour and the ratio of slag produced to carbon in the charge.

9. A method as claimed in claim 8, increasing the degree of preheating of the blast and the amount of water vapour added to the blast in a related manner above the amounts required, as shown in FIG. 4, for reducing and controlling the temperature developed at the top of the furnace.

10. In the method of operating a lead blast-furnace in which materials containing lead in oxidized form of high lead content and carbonaceous reducing fuel are charged at the top of the furnace, molten lead is tapped from the bottom of the furnace, and a molten slag, of low lead content but containing any oxides present in the charge which are less reducible than the oxidized lead, is also removed from the bottom of the furnace, the improvement in combination therewith which comprises;

(a) charging to the furnace a weight of lead in oxidized form greater than the weight of slag tapped from the furnace;

(b) introducing into the charge at the bottom of the furnace a preheated air blast of controlled volume and temperature;

(0) introducing with the preheated air blast water vapour of controlled volume and temperature into the bottom of the charge; and

(d) controlling the volume and temperature of the preheated air blast and the water vapour contained therein supplied to the bottom of the charge to amounts sufliciently high for avoiding objectionable high temperature build-up at the top of the charge to inhibit the tendency of the charge to form bridges of the charge across the furnace with consequent uneven descent and hanging-up of the descending charge and for avoiding the customary necessity of diluting the furnace charge with inert material to keep the top portion of the charge from getting too hot.

11. Method according to claim 10, which comprises;

(a) introducing into the furnace charge preheated air with a higher volume concentration of water vapour and at somewhat higher temperature than required to prevent the temperature near the top of the furnace charge from becoming so high as to cause hanging-up of the charge, the relation between the said higher volume concentration of water vapour and the said higher temperature being as shown in FIG. 4; and

(b) controlling the volumes of air and water vapour and the temperatures of the air and water vapour for effecting a further economy in the consumption of 19 20 carbonaceous reducing fuel used per unit of oxidized OTHER REFERENCES lead reduced to lead- Ostrowski et al.: Journal of Metals, v01. 13, No. 1,

References Cited by the Examiner January 1961 pages 25-30 UNITED STATES PATENTS 5 DAVID L. RECK, Primary Exam ner.

2,816,022 12/1957 Morgan et a1 7577 BENJAMIN HENKIN, Examiner.

FOREIGN PATENTS H. W. CUMMINGS, Assistant Examiner.

203,185 9/1957 Australia. 

1. IN THE METHOD OF OPERATING A LEAD BLAST-FURNACE IN WHICH MATERIALS CONTAINING LEAD IN OXIDIZED FORM AND CARBON ARE CHARGED AT THE TOP OF THE FURNACE, MOLTEN LEAD IS TAPPED FROM THE BOTTOM OF THE FURNACE, AND A MOLTEN SLAG, OF LOW LEAD CONTENT BUT CONTAINING ANY OXIDES PRESENT IN THE CHARGE WHICH ARE LESS REDUCIBLE THAN LEAD OXIDE, IS ALSO REMOVED FROM THE BOTTOM OF THE FURNACE, THE IMPROVEMENT IN COMBINATION THEREWITH WHICH COMPRISES CHARGING TO THE FURNACE A WEIGHT OF LEAD IN OXIDIZED FORM GREATER THAN THE WEIGHT OF SLAG TAPPED FROM THE FURNACE, THE WEIGHT RATIO OF LEAD IN OXIDIZED FORM TO CARBON IN THE CHARGE EXCEEDING 8.8; SUPPLYING AN AIR BLAST AT THE BOTTOM OF THE FURNACE CONTAINING AT LEAST 4% BY VOLUME OF WATER VAPOUR; PREHEATING THE WATER VAPOUR TO AT LEAST 200*C.; AND CONTROLLING THE AMOUNT OF WATER VAPOUR IN THE BLAST TO PREVENT AN EXCESSIVELY HIGH TEMPERATURE BEING GENERATED NEAR THE FURNACE TOP TO INHIBIT THE TENDENCY OF THE CHARGE TO FORM BRIDGES OF THE CHARGE ACROSS THE FURNACE WITH CONSEQUENT UNEVEN DESCENT AND HANGING-UP OF THE DESCENDING CHARGE AND FOR AVOIDING THE CUSTOMARY NECESSITY OF DILUTING THE CHARGE WITH INERT MATERIAL TO KEEP THE TOP PORTION OF THE CHARGE FROM GETTING TOO HOT. 